The present invention relates to vascular repair devices, and in particular to intravascular stents, which are adapted to be implanted into a patient's body lumen, such as a blood vessel or coronary artery, for the treatment of unstable or vulnerable, human atherosclerotic plaque.
Currently, the treatment of unstable or vulnerable plaque presents a significant therapeutic challenge to medical investigators. Vulnerable plaque is characterized by a basic lesion which is a raised plaque beneath the innermost arterial layer, the intima. Atherosclerotic plaques are primarily composed of varying amounts of long chain extracellular matrix (ECM) proteins that are synthesized by smooth muscle cells. The other primary lesion component of atherosclerotic plaque includes lipoproteins, existing both extracellularly and within foam cells derived primarily from lipid-laden macrophages. In a more advanced lesion, a necrotic core may develop, consisting of lipids, foam cells, cell debris, and cholesterol crystals, and myxomatous configurations with crystalline lipid forms. The necrotic core is rich in tissue factor and quite thrombogenic, but in the stable plaque it is protected from the luminal blood flow by a robust fibrous cap composed primarily of long chain ECM proteins, such as elastin and collagen, which maintain the strength of the fibrous cap. The aforementioned plaque represents the most common form of vulnerable plaque, known as a fibroatheroma. Histology studies from autopsy suggest this form constitutes the majority of vulnerable plaques in humans. A second form of vulnerable plaque represents a smaller fraction of the total, and these are known as erosive plaques. Erosive plaques generally have a smaller content of lipid, a larger fibrous tissue content, and varying concentrations of proteoglycans. Various morphologic features that have been associated with vulnerable plaque, include thinned or eroded fibrous caps or luminal surfaces, lesion eccentricity, proximity of constituents having very different structural moduli, and the consistency and distribution of lipid accumulations. With the rupture of fibroatheroma forms of vulnerable plaque, the luminal blood becomes exposed to tissue factor, a highly thrombogenic core material, which can result in total thrombotic occlusion of the artery. In the erosive form of vulnerable plaque, mechanisms of thrombosis are less understood but may still yield total thrombotic occlusion.
Although rupture of the fibrous cap in a fibroatheroma is a major cause of myocardial infarction (MI) related deaths, there are currently no therapeutic strategies in place to treat these lesions that could lead to acute MI. The ability to detect vulnerable plaques and to treat them successfully with interventional techniques before acute MI occurs has long been an elusive goal. Numerous finite element analysis (FEA) studies have proved that, in the presence of a soft lipid core, the fibrous cap shows regions of high stresses. Representative of these studies include the following research articles, each of which are incorporated in their entirety by reference herein: Richardson et al. (1989), Influence of Plaque Configuration and Stress Distribution on Fissuring of Coronary Atherosclerotic Plaques, Lancet, 2(8669), pp. 941–944; Loree et al. (1992), Effects of Fibrous Cap Thickness on Circumferential Stress in Model Atherosclerotic Vessels, Circulation Research, 71, pp. 850–858; Cheng et al. (1992), Distribution of Circumferential Stress in Ruptured and Stable Atherosclerotic Lesions: A Structural Analysis With Histopathological Correlation, Circulation, 87, pp. 1179–1187; Veress et al. (1993), Finite Element Modeling of Atherosclerotic Plaque, Proceedings of IEEE Computers in Cardiology, pp. 791–794; Lee et al. (1996), Circumferential Stress and Matrix Metalloproteinase 1 in Human Coronary Atherosclerosis: Implications for Plaque Rupture, Atherosclerosis Thrombosis Vascular Biology, 16, pp. 1070–1073; Vonesh et al. (1997), Regional Vascular Mechanical Properties by 3-D Intravascular Ultrasound Finite-Element Analysis, American Journal of Physiology, 272, pp. 425–437; Beattie et al. (1999), Mechanical Modeling: Assessing Atherosclerotic Plaque Behavior and Stability in Humans, International Journal of Cardiovascular Medical Science, 2(2), pp. 69–81; C. Feezor et al. (2001), Integration of Animal and Human Coronary Tissue Testing with Finite Element Techniques for Assessing Differences in Arterial Behavior, BED-Vol. 50, 2001 Bioengineering Conference, ASME 2001; and C. Feezor et al. (2003), Acute Mechanical Response Of Human Coronary Fibroatheromas To Stenting, 2003 Summer Bioengineering Conference, Key Biscayne, Fla., pp. 167–168. Further, these studies have indicated that such high stress regions correlate with the observed prevalence of locations of cap fracture. Moreover, it has been shown that subintimal structural features such as the thickness of the fibrous cap and the extent of the lipid core, rather than stenosis severity are critical in determining the vulnerability of the plaque. The rupture of a highly stressed fibrous cap can be prevented by using novel, interventional, therapeutic techniques such as specially designed stents that redistribute and lower the stresses in the fibrous cap.
Stents are generally tubular-shaped devices which function to hold open a segment of a blood vessel, coronary artery, or other body lumen. They are also suitable for use to support and hold back a dissected arterial lining which can occlude the fluid passageway therethrough.
Currently, there are no known effective methods for treating vulnerable plaque. Conventional stents and stent delivery systems have been used to treat vulnerable plaque. Various means have been described to deliver and implant stents. One method frequently described for delivering a stent to a desired intraluminal location includes mounting the expandable stent on an expandable member, such as a balloon, provided on the distal end of an intravascular catheter, advancing the catheter to the desired location within the patient's body lumen, inflating the balloon on the catheter to expand the stent into a permanent expanded condition and then deflating the balloon and removing the catheter.
One problem with conventional stents and stent delivery systems is that the conventional stent is typically inflated at a high pressure (e.g., above about 8 atm or 117 psi or higher) in order to reach a nominal dimension (e.g., 3.0–3.5 mm diameter). The required high pressure in turn may cause rupture of the vulnerable plaque and may lead to an acute MI. In addition, conventional stents and stent delivery systems are often designed to keep open a blood vessel's lumen or to provide a rigid support for the vessel. Thus, the stent imparts a certain amount of pressure or force upon the vessel to perform these functions. Using a conventional stent that is designed mainly to keep open the lumen imparts too much radial force and pressure upon the vulnerable plaque causing the vulnerable plaque or the fibrous cap of the vulnerable plaque to rupture.
What has been needed and heretofore unavailable is a stent that can be used to treat a vulnerable plaque by reducing the outward radial stresses exerted by the stent. The present invention satisfies this need and others.
Invention Summary
The present invention is directed to an intraluminal stent that can be used to treat a lesion with vulnerable plaque by reducing the outward radial stresses exerted by the stent. Furthermore, the intraluminal stent can be deployed at a substantially low pressure to minimize trauma to the vulnerable plaque.
In one exemplary embodiment, an intraluminal stent comprises a plurality of planar rings aligned along a common longitudinal axis, wherein each ring is defined by a circumference entirely contained within a plane. The circumferential planes containing the plurality of rings are preferably inclined relative to a plane perpendicular to the longitudinal axis. There is further a plurality interconnecting members having a length extending parallel to the common longitudinal axis and joining adjacent rings.
In various alternative embodiments, the intraluminal stent comprises a first end section, and middle section, and a second end section wherein only the first end section includes at least one ring having the inclined circumferential plane, or wherein only the first and the second end sections include at least one ring having the inclined circumferential plane. Alternatively, the intraluminal stent may have only the first end section and the middle section including at least one ring having the inclined circumferential plane, or wherein only the middle section includes at least one ring having the inclined circumferential plane.
In still further alternative embodiments, the intraluminal stent may have the inclined circumferential planes inclined at an angle α defined by about 30°≦α≦45° including all angles between those ranges relative to a plane perpendicular to the longitudinal axis of the stent. The inclined orientation of the rings gives the stent sufficient radial strength for scaffolding yet the stent exerts gentle pressure on the vulnerable plaque. Also, the intraluminal stent may have interconnecting members wherein they form two columns disposed about 180° circumferentially apart or three columns at 120° apart. The use of planar inclined rings and interconnecting members minimize bunching of the rings when deployed or uncontrolled tilt or attitude in the depolyed rings as sometimes seen in coiled spring stents.
The intraluminal stent includes rings that are preferably formed from a serpentine strut pattern having alternating vertices and arms, wherein at least some of the vertices at a peak include a reduced cross-section, a reduced mass section, a void, and/or a groove for reduced strength. Further, the intraluminal stent may include rings having a serpentine strut pattern having alternating vertices and arms wherein at least some of the vertices at a peak of the vertex is radially thinner than the remainder of the vertices for reduced strength, or is narrower in a circumferential width than the remainder of the vertices for reduced strength. Also, the arms but not the vertices may have increased mass, girth, thickness, width, or any combination thereof for improved radiopacity.
The rings of the present invention stent in one embodiment are plastically deformed when balloon expanded if the stent is made from a rather inelastic metal. Typically, the balloon expandable stent is made from a stainless steel alloy or similar material.
The stent may be formed by laser cutting the pattern of rings and links from a tube. The stent also may be formed by laser cutting a flat metal sheet into a pattern of the rings and links, and then rolling the pattern into the shape of the tubular stent. The longitudinal seam where the edges of the sheet meet is then welded or otherwise joined.
Other features and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the features of the invention.